1. Field of the Invention
The present invention relates to an exhaust nozzle extending to the rear of an engine and constituting an exhaust flow path, and to a method for changing an exhaust flow path whereby a cross-sectional shape of an exhaust flow path extending to the rear of the engine is altered, and in particular, to an exhaust nozzle and a method for changing an exhaust flow path suitable for an engine of a supersonic aircraft.
2. Description of the Related Art
When high-pressure fluid, such as exhaust from an engine, is discharged from an exhaust nozzle, which forms an exhaust flow path, a high-speed fluid is created at the end of the nozzle, this high-speed fluid spreads while mixing with surrounding fluid according to a speed gradient with respect to the surrounding fluid, and noise is generated in this course of this mixing action.
In particular, in the exhaust portion of a jet engine in an aircraft or the like, the noise has high energy across a broad frequency band, and areas adjacent to an airport and the aircraft passengers suffer barely acceptable effects due to exposure to noise, especially during take-off and landing.
Therefore, it is conceivable that noise during take-off and landing could be reduced by restricting the mixing and spreading of the high-speed fluid and the surrounding fluid, through providing a movable portion in the exhaust nozzle to alter the shape and cross-sectional surface area of the exhaust flow path.
For example, in the technology revealed in Japanese Patent Application Publication No. 2007-285245, in order to reduce noise, the state of mixing of a core stream and a bypass stream is altered by composing a rear end of a core flow path of an exhaust nozzle attachably and detachably, or in a passively movable fashion, and disposing same to the upstream side of a rear end of a bypass flow path in order to.
Furthermore, in Japanese Patent Application Publication No. 2008-144764, in order to reduce noise, an opening is provided between a core flow path and a bypass flow path of an exhaust nozzle, and by opening and closing the opening with a passive or active movable mechanism, the state of mixing of the core stream and the bypass stream is altered.
Moreover, the technology is also known, according to which the state of mixing is changed by providing notch-shaped projections on an inner surface of an exhaust nozzle, in order to achieve noise reduction by modifying only the cross-sectional shape of the exhaust flow path of the exhaust nozzle without changing the path of a core stream and a bypass stream (see Tsutomu OISHI, “Jet Noise Reduction by Notched Nozzle on Japanese ECO engine project”, AIAA Paper 2010-4026, etc.).
Furthermore, with a jet engine for an aircraft, if the cruising speed exceeds the speed of sound, as in a supersonic aircraft, then the propulsion efficiency is improved by providing a portion in which the cross-sectional area of the exhaust flow path gradually increases, toward the rear end side from a position where the cross-sectional area is smallest, but if the cruising speed is equal to or lower than the speed of sound, then the propulsion efficiency may be reduced if the cross-sectional area of the exhaust flow path is increased toward the rear end side from the position where the cross-sectional area is smallest.
Therefore, it is known that in supersonic aircraft, a movable portion is provided in the exhaust nozzle to alter the shape and cross-sectional area of the exhaust flow path, so as to obtain efficient propulsion during take-off and landing or when cruising at both subsonic speeds and supersonic speeds.
For instance, as shown in FIG. 11, Gordon C. Oates, “Aircraft Propulsion Systems Technology and Design”, 301-303 discloses a convergent-divergent nozzle form (A); a plug nozzle form (B); and an axially asymmetrical nozzle form (C).
With the convergent-divergent nozzle form (A), a mechanism to open and close the nozzle end is provided, thereby altering the cross-sectional area of the nozzle end.
With the plug nozzle form (B), a mechanism is provided for expanding and contracting the core portion of the nozzle tip section in the radial direction, thereby changing the cross-sectional area of the tip section.
In an axially asymmetrical nozzle form (C), the cross-sectional area of the tip section is altered by combining a fixed wall and a movable wall, and displacing the movable wall in a radial direction in the form of a flap.
Moreover, with the object of reducing noise and improving efficiency when cruising at supersonic speeds, in Japanese Patent Application Publication No. H7-208262 and Japanese Patent Application Publication No. 2007-218255, an exhaust nozzle, including a bypass flow path, is constituted by a plurality of active movable mechanisms, and the shape and cross-sectional area of an exhaust flow path are altered, or a path of a core stream and a bypass stream are changed so as to alter the state of mixing of the core stream and the bypass stream.
The technology disclosed in Japanese Patent Application Publication No. 2007-285245 and Japanese Patent Application Publication No. 2008-144764 and Tsutomu OISHI, “Jet Noise Reduction by Notched Nozzle on Japanese ECO engine project”, AIAA Paper 2010-4026 makes no mention at all of efficiency when cruising at supersonic speeds, and if these common technologies are applied to a supersonic aircraft, then in order to improve efficiency when cruising at supersonic speeds, in addition to the movable mechanism of the exhaust nozzle, increases in complexity and size are required.
Furthermore, it is also necessary to consider whether or not an improvement in propulsion is obtained which is sufficient to compensate for the increase in complexity and size, and therefore it has been difficult to improve efficiency in a practicable fashion, simply by combining the structure for noise reduction disclosed in Japanese Patent Application Publication No. 2007-285245 and Japanese Patent Application Publication No. 2008-144764, and the like, and the mechanism for improving efficiency during supersonic cruising disclosed in Tsutomu OISHI, “Jet Noise Reduction by Notched Nozzle on Japanese ECO engine project”, AIAA Paper 2010-4026, and the like.
The technology disclosed in Japanese Patent Application Publication No. H7-208262 includes a pair of first flaps A which can swing about a first horizontal axle, a pair of second flaps B which can swing about a second horizontal axle, a pair of lobe-shaped mixers M which are pivotally mounted on a third horizontal axle at the downstream end of the first flap A and which deploy horizontally in a linear shape, and a pair of third flaps C which can swing about a fourth horizontal axle, wherein the mixers M have a fifth horizontal axle, this fifth horizontal axle is coupled to the fourth horizontal axle via a link, and the fourth horizontal axle is provided movably along a guide provided on the side wall.
Furthermore, in the technology disclosed in Japanese Patent Application Publication No. 2007-218255, the first exhaust nozzle and the second exhaust nozzle are composed in such a manner that the variable cross-sectional area can be manipulated, the two nozzles have corresponding actuators which are coupled suitably to an engine controller in the form of a digital computer, and the engine controller suitably adjusts the discharge flow rate range according to the requirements for efficient operation of the engine in the operating cycles of the aircraft and the flight envelope.
These compositions are complex, and by providing these mechanisms, the weight is increased, and therefore even if an increase in propulsion that compensates for this weight increase is achieved, the improvement in efficiency during supersonic cruising is slight, and there is a problem in that it is impossible to avoid increase in manufacturing costs due to increased complexity and size, and increased labor time due to maintenance and inspection.